1. Field of the Invention
The present invention relates to a flow control device and more particularly to modular flow control device with deployable flow effectors. The present invention further relates to a method of operating the flow control device.
2. Technical Background
In numerous aeronautical and hydrodynamic applications it is desirable to control the flow of fluid across a surface. As fluid flows over a flow surface, like air over an airplane wing or forebody, air over turbine engine blades, or water around the hull of a ship or a submarine, it forms a fluid boundary layer at the surface. The fluid boundary layer is a thin layer of viscous flow exhibiting certain pressure variations that affects the operation of the vehicle surface. The pressure variations within the fluid flow inside the boundary layer directly affect the performance, including the maneuverability and stability, of the vehicle. The oscillation of the pressure variation within the boundary layer correlates the separation and attachment of the fluid flow to the solid surface. The point at which the fluid boundary layer separates from the flow surface is related to the angle of attack (AoA) of the flow surface. In the case of a wing, if separation is too near the leading edge, the wing stalls and the aircraft looses lift and the pilot looses control.
For airplanes the air flowing above and below the wing at different speeds creates the lift necessary to raise or elevate the plane off the ground. The wing causes the air to flow across the upper surface of the wing at a speed faster than the speed of air flowing across the bottom surface. The faster airflow across the upper surface of the wing creates a reduced pressure region known as suction peak along the upper surface of the wing. Due to the generally flat lower surface, a high-pressure zone is created along the lower surface of the wing, thereby generating a net upward force. With a high angle of attack, such as during a steep ascent at takeoff, or a steep descent at landing, there is a tendency of the air flow passing across the upper surface of the wing to become destabilized and separate from the wing. This separation of fluid flow leads to disastrous results because the suction peak on the upper surface of the wing is diminished, and the lift is dramatically reduced while the drag is substantially increased. Therefore, it is beneficial to delay flow separation to the highest angle of attack possible to increase the lift and reduce the drag. To achieve the highest angle of attack possible, there is a need for a means to detect and delay separation via reattaching the flow to the wing surface.
Flow control devices have been employed to control fluid boundary layer dynamics and counteract the boundary layer separation point. These devices are categorized as passive, requiring no auxiliary power, or active, requiring energy expenditure, or reactive, requiring energy expenditure and a feedback control loop. Passive devices, such as fixed vortex generators, tapered fins, scoops, flow-jet injectors and minidomes, protrude into or through the fluid boundary layer to enhance the mixing of fluid flow and thereby control fluid boundary layer dynamics. Passive devices, which involve the presence of a device continually protruding from the flow surface even with no boundary layer flow separation, such as when cruising at a given elevation, leads to increased drag on the flow surface resulting in increased fuel consumption and reduced efficiency of the air vehicle. In addition, with military aircraft the protruding passive flow control devices produce a radar signature compromising the stealth capability of the aircraft.
Active devices, such as synthetic jets, wall jets, active vortex generators, etc., which use auxiliary power for actuation, enhances the mixing of low momentum fluid flow within the boundary layer with high momentum fluid flow outside the boundary layer to prevent separation and delay stall. Most of these active devices operate in an open-loop mode and are slow and relatively unresponsive. Also, while active devices such as mechanical vortex generators can be responsive they also have some limitations. Two types of mechanical vortex generators are deployable flow control devices and pressure active regions. Pressure active regions function by creating vortices using suction or air pressure at various points along the aircraft surface. Pressure active regions due to their nature require, in addition to an electrical system, a separate pneumatic piping system and a central pressure or vacuum source. With these systems, the vacuum or pressure required to generate a vortex to reattach the flow is fairly high at high speeds, which increases the power requirements, weight and the cost of the system. Complications and significant expense can occur with these systems if the lengthy pneumatic systems develop a leak or if the system is activated at a high frequency. Additionally, the openings in the surface of the wing or airfoil may lead to drag and unwanted surface effects. These requirements and complications make such systems bulky, complicated, heavy and expensive. U.S. Pat. No. 6,302,360 to Ng is an example of such a system with some of these limitations. On the other hand, deployable flow effectors have been described in U.S. Pat. No. 6,302,360 to Ng as having other limitations. These are their being relatively slow in response, and high in weight.
The technical advantage of a predictive system over other systems is that in predictive systems, the onset of flow separation can be detected before it occurs and means of control can be employed to avoid separation, thereby eliminating any losses due to flow separation. Current systems such as that described in U.S. Patent to Palmer are illustrative of the state, and limitations of the prior art. Palmer discloses a method for measuring the air pressure differentials between two or more sensors to evaluate certain critical flight parameters. This evaluation, though, provides information about the present flight conditions, for instance, locating a stagnation point on the flow surface. The method disclosed in Palmer cannot determine the incipience of a stagnation point or separation point at any location on the flow surface. It therefore, is not predictive, but historical. As such, any change or modification of the flow surface, for instance the change in the AoA, is reactive not proactive.
In view of the foregoing inherent disadvantages with presently available passive or active flow control systems, it has become desirable to develop a method and a device to be able to predict flow surface dynamic occurrences, including stall, upon their incipience, in order to allow for proactive change or modification of the flow surface to proactively take into account and/or preclude such flow surface dynamic occurrences. Additionally there is a need to develop a reactive modular system operating in a closed-loop mode that can be installed or retrofitted relatively easily on existing airfoils. The device should be adapted with controllers and pressure sensors to enable local measurement and feedback for controlling the active deployable flow effectors to reattach the airflow to the airfoil.